In recent years and with prolongation in average life span, diseases such as senile dementia with memory disturbance are causing medically and socially great problems.
Dementia is a condition that cerebral functions once developed have been continually retarded as regard to memory, cognition and thinking, resulting in problems in ordinary social life. Alzheimer's disease, cerebrovascular dementia and mixture thereof amount to eight- or nine-tenths of underlying diseases for senile dementia, a core symptom of which is memory impairment. Known with respect to Alzheimer's disease are the facts that the activity of choline acetyltransferase (ChAT), which is an acetylcholine sythesizing enzyme in the cerebral cortex, is lowered in comparison with normal control group of the same age [Bowen et al., Brain, 99, 459 (1976)] and that neurons in the nucleus basalis of Meynert, which is the nuclei of origin in cholinergic nerve of the cerebral cortex, are eminently exfoliated [Whitehouse et al., Science, 215, 1237–1239 (1982)]. Moreover, it is known, for example, that retarded cognitive function in terms of mental test score is related with lowered ChAT activity of the cerebral cortex [Perry et al., Br. Med. J. 25, 1457–1459 (1978)] and that scopolamine, which is a pharmacologically muscarinic receptor antagonist, causes amnesia [Drachman, Neurology, 27, 783–790 (1977)]. Set up against these backgrounds was a cholinergic hypothesis to the effect that memory is deeply linked with cholinergic nerve [Bartus et al., Science, 217, 408–417 (1982)]; nowadays, approaches based on cholinergic hypothesis have been made on development of curative medicines for senile dementia. Especially, experimental animal models with learning and memory disturbance induced by anti-cholinergic chemicals (e.g., scopolamine) have been widely utilized in quest of medicines effective for learning and memory disturbance due to generally various causes of diseases (e.g., senile dementia including Alzheimer's disease).
Development of curative medicines effective against senile dementia has been strongly demanded; up to the present, antidementia medicines such as linopirdine, tacrine or aricept have been proposed and some of them have been marketed.
However, none of developed and marketed antidementia medicines are satisfactory in improvement and remedy of dementia symptoms. There are still, therefore, strong demands on development of more effective antidementia medicines.